A man inspects a damaged building after what activists said was an airstrike by forces loyal to Syria's President Bashar al-Assad in the Duma neighborhood of Damascus December 25, 2014

AMMAN - At least 45 civilians were killed and some 175 wounded when aircraft bombed a northern Syrian city controlled by Islamic State, as Bashar al-Assad's government stepped up air raids, residents and a monitoring group said on Friday.

Helicopters and war planes dropped barrel bombs - steel drums full of shrapnel and explosives - on residential and industrial areas in the city of al Bab and neighbouring Qabaseen, northeast of Aleppo, on Thursday and overnight, locals said.

"People were going about scraping a living and there were no armed groups in the market, only poor people. Why is Assad killing us? May God bring vengeance on him," said Yousef al-Saadi, a resident of Qabaseen and a volunteer with the local civil defence group who was contacted on Skype.

Syrian state media did not report the strikes on al Bab, a city of around 100,000 people that has been a target of heavy government strikes since the start of U.S- led military campaign against Islamic State, an Islamist militant group, in Syria in late September.

The Syrian Observatory for Human Rights said 37 civilians were killed. The British-based group, which gathers information from a variety of sources, said there had been an increase in air raids by the Syrian military across rebel held areas in the last three days.

It said at least 110 civilians had been killed in more than 470 air strikes on rebel held areas in Syria in the last 72 hours, including towns in insurgent-held eastern suburbs of the capital Damascus, where the army has stepped up a two-year campaign to retake the area.

Eleven civilians, most women and children, were killed by loyalist snipers when they were trying to leave Zebdin, a besieged rebel-held neighbourhood in the rural outskirts of Damascus, the Observatory said.

"There have been unprecedented air raids across Syria in the last three days where the regime seeks to make gains on the ground to improve its negotiating stance in future political talks," Rami Abdul Rahman, head of the Observatory, told Reuters.

Syrian media said it had repulsed "terror attacks" across rebel-held areas and inflicted losses against foreign jihadists but gave no figures on civilian casualties from its air raids.